There has been a strong demand in recent years for communicating the contents of printed matters visually in a more impactful manner by utilizing many photographs, illustrations and/or colors (hereinafter referred to as “visualization”). On the other hand, there is also an increased demand for reducing the weight of printed matters to preserve natural resources and save the costs of shipping. These two demands are mutually contradictory, because high-grade coated papers suitable for visualization are generally associated with higher basis weight of the base paper, higher coating weight and higher price, which does not meet the requirements for lightweight, low-cost papers. For the above reason, the market is demanding technologies that can provide lightweight and low-cost “low-grade” coated papers that offer lower basis weight and coating weight, while achieving the quality of higher-grade papers.
One of the important factors that affect the quality of coated papers is wrinkling, after drying section of web offset printing. In the drying process of web offset printing, moisture in the paper evaporates at different rates in the non-image area and the image area covered with ink. In the non-image area, the speed of moisture evaporation is faster than that in the image area. As a result, the non-image area starts to shrink first and the paper wrinkles in the machine direction.
This phenomenon of wrinkling becomes prominent on papers with a lower basis weight and therefore poses a major impediment in the efforts to develop lightweight coated papers.
Since wrinkling is caused by a differential evaporation speed of moisture in the paper, reducing the moisture content of the coated paper before printing is an effective way to suppress wrinkling.
However, although reducing the moisture content of the coated paper reduces the degree of wrinkling to some degree, wrinkling cannot be reduced to a sufficient level this way.
Also, as published in Japanese Patent Application Laid-open No. 11-350391, a method is proposed for suppressing the shrinking behavior of paper by coating on both sides of the base paper a layer of polyvinyl alcohol (PVA) with a saponification level of 85 mol percent or more in such a way that the dry weight of the paper will become 1 to 6 g/m2. However, PVA has high viscosity and poor printability and is therefore unsuitable for use in a continuous printing operation. In fact, PVA-coated papers printed on continuous printing lines have often exhibited poor blister resistance.